A. Field of the Invention
The invention relates to a damper mechanism, particularly a damper mechanism for damping torsional vibrations in a power transmission system.
B. Description of the Background Art
A clutch disk assembly used in, for instance an automotive vehicle, is typically installed in a clutch mechanism such that the clutch disk assembly may be used in clutch engagement and clutch dis-engagement operations for transmitting torque from a flywheel to a transmission input shaft. The clutch disk assembly preferably also includes a vibration dampening function for absorbing and damping vibration transmitted from the flywheel. Generally, vibrations of a vehicle include idling noises (rattle), driving noises (acceleration/deceleration rattle and muffled noises) and tip-in/tip-out (low frequency vibrations). The clutch disk assembly has the above damper function for removing these noises and vibrations.
The idling noises are rattling noises which occur from a transmission when the transmission is in a neutral position, e.g., during waiting at traffic signals with clutch pedal off. This rattling occurs due to the fact that an engine torque is low in an engine idling range and engine combustion causes large torque variations in the idling range. In this state, gear contact occurs between an input gear and a counter gear of a transmission, and thereby noises are produced.
The tip-in/tip-out low frequency vibrations are large longitudinal vibrations of a vehicle which occur when a driver rapidly depresses or releases an accelerator with the clutch in an engaged, torque transmitting condition. If rigidity of a drive transmission system is low, torque transmitted to wheels is transmitted or reflected from the wheels back through the drive train creating large oscillations of torque.
In a state where no torque is transmitted (zero torque transmission), for instance during idling, the dampening characteristics of most clutch disk assemblies is such that idling vibrations cannot be adequately dampened creating corresponding noises, therefore, a low torsional rigidity is preferable in this region of zero torque transmission. Contrarily, it is necessary to maximize the rigidity of the torsion characteristics of the clutch disk assembly for suppressing the longitudinal vibrations of the tip-in/tip-out.
For overcoming the above problems, a clutch disk assembly which uses two kinds of springs for achieving vibration dampening characteristics in two separate stages has been developed. The structure of this clutch disk assembly includes three rotary members adapted to undergo relative rotary displacement with respect to one another. A first spring having a low rigidity elastically couples first and second rotary members. A second spring having more rigid or stiff characteristic elastically couples a third rotary member and the second rotary member. The clutch disk assembly is configured to have a low torsional rigidity and a low hysteresis torque in the first stage where the first spring is compressed. Vibrations exhibiting small angular displacement having a low torsion angle are dampened and therefore the clutch disk assembly can achieve an effect of preventing noises during idling. Since the torsional rigidity and the hysteresis torque are high in the second stage of a high torsion angle due to the stiffness of the second spring, the longitudinal vibrations at the time of tip-in/tip-out can be effectively damped.
Such a damper mechanism is already known where operation of a high hysteresis torque generating mechanism (friction generating mechanism) in the second stage is at least partially prevented when minute vibrations occur thereby allowing dampening of minute vibrations by a low hysteresis torque.
The angular displacement within the second stage of operation in which a large friction mechanism does not operate is very small and, e.g., about 2 degrees. This region of the second stage can be provided in the positive second stage, in which the input rotary member rotates or twists in the torque transmission direction (positive rotational direction) relatively to the output rotary member, and the negative second stage, in which the relative rotation occurs in the opposite direction (negative rotational direction). In the prior art, the same structure is used for limiting the operation of the large friction mechanism in both the positive and negative portions of second stages. Therefore, the torsion characteristics in positive and negative rotation directions, in which a high hysteresis torque does not occur in response to minute vibrations, have equal circumferential angles with respect to one another.
However, the angular displacement in the positive rotation direction within the second stage of operation which exhibits low hysteresis torque must be sufficiently large to prevent generation of high hysteresis torque in response to engine torque vibrations during normal driving. However, if in the positive rotation direction the low hysteresis torque is large, the negative direction characteristics of low hysteresis torque may be excessively large. Specifically, if the angular displacement in the negative rotation direction for generation of low hysteresis torque is large, it may be impossible to generate high hysteresis torque on the opposite sides of the resonance frequency during deceleration, resulting in a large vibration peak.